Advanced magnetic recording media are required to have increased recording density. Among others, research work has been focused on magnetic recording media using ferromagnetic metal thin films based on cobalt with nickel or similar elements added since they have high saturation magnetic flux density and high coercive force.
Magnetic recording media of this type are fabricated by various methods including oblique evaporation also known as deposition method of continuously varied incidence. One preferred mode of oblique evaporation is by depositing two or more layers of ferromagnetic metal thin film on a non-magnetic substrate to form a multi-layer structure thereon. In the oblique evaporation method, each layer of ferromagnetic metal thin film is prepared by furnishing vapor of ferromagnetic metal by a vapor phase method such as evaporation, and directing the ferromagnetic metal vapor toward the non-magnetic substrate surface at a certain angle. A second or upper layer is obtained by growing columnar grains of ferromagnetic metal in a direction transverse to the direction of growth of columnar grains of ferromagnetic metal of an underlying layer. Reference is made to the following publications.
______________________________________ Japanese Patent Publication (JP-B) Nos. 26891/1981 42055/1981 37528/1985 21254/1988 Japanese Patent Application Kokai (JP-A) Nos. 603/1979 147010/1979 94520/1981 3233/1982 30228/1982 13519/1982 141027/1982 41028/1982 141029/1982 143730/1982 143731/1982 147129/1982 14324/1983 50628/1983 76025/1985 110333/1986 187122/1986 10315/1988 13117/1988 14317/1988 14320/1988 39127/1988 ______________________________________
The multi-layer structure improves coercive force and other electromagnetic properties as well as mechanical properties. There is a demand for further improvement.
Regarding magnetic recording media of the longitudinal recording type, the inventors found that there was left a room for improvement in electromagnetic properties and durability, because the directions of growth of columnar grains in ferromagnetic metal thin films and their correlation and the thicknesses of such films and their correlation were not fully studied.
One approach to these problems was proposed in JP-A 9015/1988 by the applicant, which is directed to a magnetic recording medium comprising two layers of Co-Ni base ferromagnetic metal thin film wherein the direction of growth of columnar grains of the upper layer intersects that of the lower layer. By a choice of thickness such that the upper layer was thinner than the lower layer, electromagnetic properties and durability were improved as well as dynamic properties. In fact, improvements in durability and dynamic properties were satisfactory, but there was a room for further improvement in electromagnetic properties.
Regarding magnetic recording medium of the same two layer type, the applicant attempted in JP-A 10314/1988 to improve electromagnetic properties and durability by adjusting the minimum incident angle. The minimum incident angle is the angle defined between the direction of incidence of metal particles in a final evaporation stage of each ferromagnetic metal thin film and a normal to the non-magnetic substrate. However, durability, especially under high-temperature, high-humidity condition and electromagnetic properties were still unsatisfactory because of a relatively greater minimum incident angle for the upper layer and because of the two layer arrangement.
Magnetic recording media having three or more layers of ferromagnetic metal thin film formed by oblique evaporation are also known. One as disclosed in JP-A 134317/1981 was unsatisfactory in electromagnetic properties or temperature and humidity resistance because the thickness correlation among the respective ferromagnetic metal thin film layers was not contemplated and the incident angle of metal particles was greater. JP-A 60205/1978 discloses a similar medium which was less resistant against corrosion because the respective layers had an approximately equal thickness of 500 to 700 .ANG. and an incident angle of ferromagnetic metal as great as 22 to 72 degrees. Moreover, in JP-A 39127/1988 and 10315/1988, the ferromagnetic metal thin film of the uppermost layer was oxidized into oxide to enhance durability, but at the sacrifice of electromagnetic properties due to the same oxide.